Abstract
Bismuth telluride has the highest figure of merit, and is thus widely employed in thermoelectric refrigeration. In this study, the method of fabrication of Bi2Te3 alloy by electrodeposition was investigated. When electrodepositing Bi2Te3 thin film, the importance of mass transport of the electroactive species has been evaluated by quartz crystal microgravity (QCM) and rotating disc electrode (RDE). Under a constant polarized condition, the composition of the deposit was confirmed to be adjustable by the diffusion-controlled method. Both QCM and RDE studies reveal the importance of supplying Bi3+ and HTeO2+ for altering the film composition and the reaction rate. Firstly, the film composition depends mainly on the surface concentrations of the active species rather than the bulk concentrations. Thus the diffusion-controlled method contributes an effective route to determine the film composition without changing the polarized condition. Secondly, when the potential is below -0.14 V vs. SCE, the deposition rate can be expressed by the relation,r (Bi-Te) = 8.569x10^-6 x [Bi3+]^0.315 x [HTeO2+]^0.245Also, we have fabricated Bi2Te3 nanowire arrays via anodic alumina membrane, which have potential applications in thermoelectric devices due to the predicted enhanced thermoelectric properties of nanowires relative to the bulk. A rotating electrode was employed to investigate the electrodeposition of Bi2Te3 nanowires. We found that mass transport of electrolytes into alumina templates of high aspect ratio plays a significant role in determining the properties of the obtained wires. Diffusion is the rate-determining mechanism of mass transport within these nano-channels. In addition to slow growing rate, the effect of mass transport causes a slight composition variation from the bottom to the top of the wires. With a rotating electrode, the composition variation along the wires can be reduced by shortening the concentration depleted zone from the bulk electrolyte to the opening of pores. The wire growing rate can consequently be increased. Moreover, the wire compositions were confirmed to be adjustable by varying the rotation speed when a thin template was used as substrate.The characterization of thermoelectric properties of electrodeposited Bi2Te3 thin films was discussed in chapter 4. The film composition was varied by our proposed diffusion-controlled method, which is related to the change of Bi3+/HTeO2+ ratios in a controlled diffusion layer. A homogeneous and dense film with precise chemical composition could thus be obtained under constant electrode polarization. Meanwhile, the sole dependence of film properties on composition change of both Te-rich and Bi-rich films were investigated. Firstly, the studies of XRD and FE-SEM show that different Te contents in deposit would lead to different dimensions of unit cell and grain sizes. The Seebeck coefficient increased apparently when the Te content was over 60 at.%Te. Te-rich films had higher carrier concentration but lower mobility than Bi-rich films. Inverse relations were observed between carrier concentration and carrier mobility and between Seebeck coefficient and conductivity. Therefore, an optimal power factor of 7×10^-4 W/m K2 was realized near the stoichiometric Bi2Te3.If heat treatment was employed on the deposited films, the Seebeck coefficient is possible to increase with the electrical conductivity due to improved crystallinity. The power factor could significantly be increased to a value of 15×10^-4 W/m K2 by heat treatment.